We have T-mobile and ATT phones. They told me, their plans have limited data, they are not concern of devices which we put sim cards in - use tablet as phones if we want. Our sim cards are regular size cards. Do you know any tablet take them? Thank you.
Here is a partial list of tablets that I think will work
Samsung Galaxy Tab 8.9 3G
HTC Jetstream
Pantech Element
Sony (P)
Some of these come in a non 3G capable model but the 3G models are available. It will not turn your tablet into a phone but you should still be able to do data. AT&T in particular would want a different data plan as the phone plans differ from the tablet plans.